lx1689 Microsemi Corporation, lx1689 Datasheet - Page 12

• Manufacturer Part Number: lx1689
• Description: Third Generation Ccfl Controller
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.1, 2006-02-27
LX1689 OPERATION
modes are employed by the LX1689. Upon power up or ENABLE
going true, Power On Delay is automatically invoked.
Immediately after termination of Power On Delay, or ENABLE
going true, strike mode is entered. After a successful strike, e.g.
lamp is ignited, run mode is entered. If ignition is unsuccessful, or
if the lamp extinguishes while running, Fault mode is entered.
Lamp ignition is determined by monitoring the lamp current
feedback voltage at pin I_SNS. Lamp current cycles are counted
from the beginning of Strike mode. If 8 or more complete cycles
occur the lamp is declared ignited. If less than 8, the lamp is
considered not ignited and Strike mode continues until ignition is
detected or strike time out is reached.
8192 x 1/f
extinguished. If at least 8 lamp current pulses are counted in each
sample, Run mode is maintained.
entered. Strike mode can be entered only once for each on/off cycle
of either V_BATT or ENABLE.
intermittent lamp failures cannot cause the module to continuously
output maximum strike voltage.
inhibited. Delay is 16384 x 1/f
clocks. The first of 16 sweeps is decoded as the power on delay
period. The subsequent 15 sweeps are used for controlling the
Ramp generator during Strike Mode. Power on delay is activated
at every V_BATT power up sequence and ENABLE sequence.
Control of the Ramp Generator frequency is switched to a DAC
output. Frequency is increased in a saw tooth fashion from normal
run value to as high as five times that value, for up to 15 sweeps.
If while strike frequency is ramping up, the over voltage set point
at OV_SNS is detected, strike frequency will freeze at that value
until either the lamp strikes or the timeout is reached. Strike Mode
is terminated by reaching 15 sweep counts or by detecting lamp
ignition. If strike is successful, Run Mode is entered.
unsuccessful, Fault mode is entered, a fault is declared and the A &
B outputs are shut off.
operate at the unloaded resonant frequency of the transformer and
lamp load. This generates the high lamp striking voltage required,
since at resonance, output voltage from the transformer will
increase to any value needed to cause ignition.
voltage divider provides output voltage feedback to the OV_SNS
pin, which freezes Strike Frequency to limit maximum output
voltage to a safe value.
Four operating modes: Power On Delay, Strike, Run, and Fault
After run mode is entered lamp current cycles are sampled every
Power ON Delay Mode
All functions are activated except that AOUT and BOUT are
Strike Mode
Entered from Power On Delay, or upon an ENABLE sequence.
The purpose of sweeping lamp frequency up during strike is to
TM
O
to determine that the lamp has not inadvertently
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
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determined by counting Ramp
Otherwise, Fault mode is
This insures that even
D E S C R I P T I O N ( C O N T I N U E D )
A capacitive
Integrated Products
®
Microsemi
If
DESIGN PROCEDURE
maximum safe output voltage, maximum strike potential is
continuously impressed across the lamp for the entire strike period.
stepped, the entire range of possible self-resonant frequencies is
covered.
panel capacitance values are no longer critical.
frequency range easily covers the self-resonant frequency of all
practical lamp assembly and transformer combinations.
V_BATT or ENABLE off and on.
its normal run value.
frequency control is immediately switched from DAC output to a
fixed reference that sets the normal run frequency. During Run
mode, the Fault Detect Counter is reset approximately every 8192 x
1/f
8 current cycles received during each period. If less than 8, the
lamp is considered extinguished and the Fault Mode is entered.
described above. In Fault Mode, the A & B output drivers are
forced low and the BRITE_C pin is driven to VDD_A to indicate
the fault condition.
ENABLE off then on, or by removing and applying V_BATT.
External load on the BRITE_C pin is limited to a filter capacitor and
single CMOS gate input.
The output of the oscillator, RAMP_C, controls all timing functions.
It must be chosen first, and will be in the range of 10K to 150K
ohms.
formula : R
that produces direct digital dimming, or a higher frequency PWM
signal that is converted to a proportional DC level by adding a filter
capacitor at the BRITE_C pin. 100% duty cycle corresponds to 1.1
volt, and 0% duty cycle corresponds to zero volts at the BRITE_C
pin. Maximum BRITE_IN input frequency for PWM inputs is 100
KHz, but when converting frequencies above 25 KHz to DC, some
accuracy is lost. The BRITE_IN input circuitry includes on-chip
active voltage clamps that ignore input voltage greater than 2.0V
and less than 0.5V. This allows the use of digital PWM input
signals where brightness is dependent only on duty cycle, with no
contribution
Since strike frequency is held constant once the LX1689 senses
Because strike frequency is ramped up rather than simply
The only way to re-initiate the strike process is to either cycle
If ignition is successful, ramp frequency immediately returns to
Run Mode
Entered only by detection of a successful Strike. Ramp generator
Fault Mode
Fault Mode may be entered from either Strike or Run Mode as
Selecting the I_R resistor value
This resistor determines the frequency of the on chip oscillator.
Driving the BRITE_IN Input
BRITE_IN can be a DC voltage, a low frequency PWM signal
O
RAMP_C frequency is twice lamp output current frequency.
. The lamp current cycle counter is monitored to insure at least
Third Generation CCFL Controller
The output frequency approximated by the following
Transformer manufacturing is simplified and parasitic
I_R
P
= 5.24E
RODUCTION
9
Fault mode may be cleared by cycling
/ FLAMP
D
ATA
OUT(Hz)
S
HEET
The 5:1 strike
LX1689
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